A three-way catalyst has been conventionally provided in the exhaust passage of an internal combustion engine in order to purify the exhaust gas discharged from the engine. It is widely recognized that the three-way catalyst has an oxygen adsorption function. Thus, when excess oxygen is contained in the gas (catalyst inflow gas) flowing into the three-way catalyst, the three-way catalyst adsorbs the oxygen and decreases NOx. When excess unburned matter is contained in the catalyst inflow gas, the three-way catalyst releases the adsorbed oxygen and purifies the unburned matter. The three-way catalyst will be referred to hereinbelow simply as “catalyst”.
The conventional air-fuel ratio control apparatus (conventional apparatus) is provided with an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor disposed upstream and downstream, respectively, of the catalyst in the exhaust passage of the engine. The conventional apparatus controls “the air-fuel ratio (of the gas mixture supplied to the engine (air-fuel ratio of the engine)” so as to match the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor (detected upstream air-fuel ratio) with the target air-fuel ratio (upstream target air-fuel ratio, target air-fuel ratio of the catalyst inflow gas). This control is called “main feedback control”.
The conventional apparatus also controls the air-fuel ratio of the engine by calculating a sub-feedback-amount and substantially changing the upstream target air-fuel ratio by the sub-feedback amount so that the output value of the downstream air-fuel ratio sensor matches the “target value corresponding to the stoichiometric air-fuel ratio” (see, for example, Patent Document 1). The air-fuel ratio control using the sub-feedback amount is called “sub-feedback control”.
Patent Document 1: Japanese Patent Application Publication No. 2009-162139